In general, this invention relates to a ternary photoinitiator system for cationically polymerizable resins. More specifically, this invention relates to photopolymerizable compositions that contain a cationically polymerizable resin and a ternary photoinitiator system that is activated upon exposure to actinic radiation. This invention also relates to methods of polymerizing such compositions using this photoinitiator system.
Epoxy-containing compounds are known to be curable using various cationic initiator systems. Smith, U.S. Pat. No. 4,256,828, describes photopolymerizable compositions that contain epoxides, an organic compound with hydroxyl functionality, and a photosensitive aromatic sulfonium or iodonium salt of a halogen containing complex ion. Hayase et al., U.S. Pat. No. 4,835,193, describe photopolymerizable epoxy resin compositions that comprise an epoxy resin and a heteropoly-acid aromatic sulfonium salt as the photocuring catalyst. In WO 95/14716 Neckers et al. describe photohardenable compositions that comprise a cationically polymerizable compound, a xanthene or fluorone dye, a hydrogen donor, and an onium salt. Palazzotto et al., U.S. Pat. No. 5,545,676, describe addition polymerization of free-radically polymerizable materials. The disclosed photoinitiator system comprises an aryliodonium salt, a sensitizer, and an electron donor having an oxidation potential less than or equal to that of p-dimethoxybenzene.
Oxman et al., U.S. Pat. Nos. 6,025,406 and 6,043,295, describe a ternary photoinitiator system for curing of epoxy resins. Oxman et al., U.S. Pat. Nos. 5,998,495 and 6,187,833, describe a ternary photoinitiator system for curing of epoxy/polyol resins. For both the epoxy resins and the epoxy/polyol resins, the ternary photoinitiator system comprises an iodonium salt, a visible light sensitizer, and an electron donor, wherein the photoinitiator system has a photoinduced potential greater than or equal to that of 3-dimethylaminobenzoic acid in a standard solution of 2.9xc3x9710xe2x88x925 moles/g diphenyl iodonium hexafluoroantimonate and 1.5xc3x9710xe2x88x925 moles/g camphorquinone in 2-butanone.
Weinmann et al., U.S. Pat. No. 6,084,004, describe compositions that undergo cationic curing and comprise a diaryliodonium compound, an alpha-dicarbonyl compound, a compound containing epoxide and/or oxetane groups, and an aromatic amine.
Briefly, and in one aspect, the invention provides a photoinitiator system for a cationically polymerizable resin. The photoinitiator system comprises an iodonium salt, a visible light sensitizer, and an electron donor compound having an oxidation potential greater than 0 and less than that of 1,4-dimethoxybenzene when measured versus a saturated calomel electrode. The photoinitiator system has a photoinduced potential less than that of 3-dimethylaminobenzoic acid in a standard solution of 2.9xc3x9710xe2x88x925 moles/g diphenyl iodonium hexafluoroantimonate and 1.5xc3x9710xe2x88x925 moles/g camphorquinone in 2-butanone.
In another aspect the invention provides a photopolymerizable composition comprising a cationically polymerizable resin, and a photoinitiator system like that described above. Optionally, the photopolymerizable composition may further comprise a free-radically polymerizable resin and/or a hydroxyl-containing material.
The cationically polymerizable resin may be selected from epoxy, oxetane, vinyl ether and spiro-orthocarbonate resins, and combinations thereof. Preferably, the cationically polymerizable resin comprises an epoxy resin, especially a silicon-containing epoxy resin, or a blend of a silicon-containing epoxy resin and an epoxy resin that does not contain silicon.
The iodonium salt may be a diaryl iodonium salt such as diaryliodonium hexafluorophosphate, diaryliodonium hexafluoroantimonate, 4-octyloxyphenyl phenyliodonium hexafluoroantimonate, 4-(2-hydroxytetradylecoxyphenyl)phenyliodonium hexafluoroantimonate, 4-(1-methylethyl)phenyl 4-methylphenyliodonium tetrakis(pentafluorophenyl)borate, and combinations thereof.
The visible light sensitizer may be selected from ketones, coumarin dyes, xanthene dyes, fluorone dyes, fluorescein dyesaminoketone dyes, p-substituted aminostyryl ketone compounds, and combinations thereof. More preferably, the visible light sensitizer is an alpha-diketone; camphorquinone is particularly preferred.
Preferred electron donor compounds for use in the invention possess one or more (and more preferably several if not all) of the following properties: (a) they are soluble in the polymerizable composition; (b) they do not absorb a significant amount of light at the wavelength of the light used to photopolymerize the composition, typically the wavelength at which the visible light sensitizer exhibits maximum absorption, by which it is meant that the electron donor compound does not detrimentally affect the performance of the visible light sensitizer; (c) they have an oxidation potential (Eox) greater than 0 but less than that of 1,4-dimethoxybenzene when measured versus a saturated calomel electrode (more preferably an oxidation potential less than about 1.35 volts, and most preferably an oxidation potential between about 0.5 and 1.34 volts); (d) they yield a photoinitiator system that has a photoinduced potential less than that of 3-dimethylaminobenzoic acid in a standard solution of 2.9xc3x9710xe2x88x925 moles/g diphenyl iodonium hexafluoroantimonate and 1.5xc3x9710xe2x88x925 moles/g camphorquinone in 2-butanone; (e) a pkb greater than 8; (f) they impart not more than a minimal amount of objectionable color to the polymerized resin; (g) they impart not more than a minimal amount of objectionable fluorescence to the polymerized resin; (h) they cause no more than a minimal amount of polymerization inhibition; (i) they improve the shelf life stability of the photopolymerizable composition; (j) they can be used in a lower effective concentration than other polymerization aids; and (k) they can increase the polymerization speed of a polymerizable composition relative to the same composition but not containing the electron donor compound (e.g., a polymerizable composition containing such an electron donor compound can cure after less than about 2 minutes exposure to a light source that generates light of a wavelength to which the visible light sensitizer is sensitive).
Preferred electron donor compounds are polycylic aromatic compounds (such as biphenylenes, naphthalenes, anthracenes, benzanthracenes, pyrenes, azulenes, pentacenes, decacyclenes, and derivatives (e.g., acenaphthenes) and combinations thereof), and N-alkyl carbazole compounds (e.g., N-methyl carbazole).
Photopolymerizable compositions according to the invention can provide a wide variety of utilities such as a photopolymerizable adhesive, a curable ink imaging layer, a silverless imaging layer, an imaging layer on a projection plate, an imaging layer on a laser plate, a hard coat layer on an optical lens, or a coating on an optical fiber. The photopolymerizable compositions of the invention are especially useful as dental materials such as dental adhesives and dental composites.
In another aspect, the invention provides a method for reducing the time needed to polymerize a cationically polymerizable resin. The method comprises the steps of:
a) providing a cationically polymerizable resin;
b) providing a photoinitiator system like that described above for the cationically polymerizable resin;
c) combining the cationically polymerizable resin and the photoinitiator system to provide a polymerizable mixture; and
d) exposing the polymerizable mixture to a light source having a wavelength and intensity to which the photoinitiator system is reactive and for a time until the polymerizable mixture attains a hard, tack-free state;
wherein the time until the polymerizable mixture attains a hard, tack-free state is less than the time required for the same polymerizable mixture, but excluding the electron donor compound, to achieve the same hard, tack-free state when exposed to the same light source.